1. Field of the Invention
The present invention relates to apparatus for resistance butt welding of pipes in constructing petroleum and gas trunk pipelines. More specifically, the invention is concerned with the construction of the apparatus placed inside pipes for resistance butt welding thereof.
Resistance butt welding apparatus find ever increasing application, especially for welding small (about 500-mm) and medium (up to 900-mm) diameter pipes of trunk pipelines.
However, employing resistance butt welding apparatus for welding large (over 900-mm) diameter pipes involves problems in attaining an accurate alignment of the pipes to be welded, which stem from the fact that larger-diameter pipes are heavier and tolerances for both their inside diameter and the taper of their inside surface are greater.
It is common knowledge that to ensure a reliable alignment of the pipes to be weld-joined, an apparatus must, firstly, offer an adequate stiffness and, secondly, effect a reliable clamping of the ends of the pipes. To meet the latter requirement, the apparatus should grip a pipe end over such a length (base) that the weight of the free end of the pipe would not affect the accuracy of aligning the pipes.
2. Description of the Prior Art
The most extensive application for welding large diameter pipes has been found by resistance butt welding apparatus with self-aligning conical clamps as described in USSR Inventor's Certificate No. 351,423 (1970). With such clamps, however, the pipes, i.e. their ends to be welded together, must meet more stringent requirements to ensure a reliable alignment.
Employing other prior art resistance butt welding apparatus, such as that disclosed in USSR Inventor's Certificate No. 292,743, gives rise to adverse effects in that the working fluid pressure is used only to actuate clamps by hydraulic cylinder rods, while the response is taken up by fixed bodies of the cylinders. There is known a pipe resistance butt welding apparatus (USSR Inventor's Certificate No. 326,031, 1970) which comprises an elongated supporting member carrying a fixed expanding clamp and a movable expanding clamp, serving to locate by current-feeding and clamping shoes the pipes to be welded, and a device for drawing the pipes together in fusion and upsetting. The expanding clamps have the form of vane-type rotary actuators with turnable vanes alternately rigidly connected to two turnable shaped rings which act through rollers on the clamping and current-feeding shoes.
Although in this apparatus the clamping and current-feeding shoes of each expanding clamp are spaced at a considerable distance from each other and provide for clamping the pipe end over a great length (base), the apparatus has nevertheless some disadvantages which cause difficulties in welding large-diameter pipes.
First of all, the above rotary actuator by itself makes the apparatus more complex and heavier. In addition, an inside surface taper tolerable in large diameter pipes causes the shoes to bear against the pipe by only a part of their surface, which impairs the dependability of clamping the pipe end and eventually lowers the weld quality. Furthermore, the rollers contacting the shaped rings are acted upon by much greater concentrated loads at points disposed on the same vertical with the point of application of the maximum force to a shoe, which causes the rollers to fail.